This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Ribonucleotide reductases (RNR) are essential to cellular organisms. They reduce ribonucleotides to deoxyribonucleotides and provide the building blocks of deoxyribonucleic acids (DNA). RNRs are divided into three different classes based on the mechanism they use for ribonucleotide reduction. Much research has been done on class I RNRs which include ribonucleotide reductase from humans and E. coli. Originally all class I RNRs were believed to utilize a diiron center and an ?essential? tyrosine radical to initiate proton coupled electron transfer. The proton coupled electron transfer generates a cysteine radical in R1 to begin ribonucleotide reduction. However, the human pathogen Chlamydia trachomatis (Ct) lacked the ?essential? tyrosine and had a phenylalanine in its place. It was later discovered by Jiang et al. that the R2 subunit of RNR from Ct featured a manganese and an iron instead of a diiron center and a tyrosine. In Ct R2 the heterobinuclear center starts as a Mn(II)/Fe(II) which is oxidized by molecular oxygen (O2) to the Mn(IV)/Fe(IV) intermediate. This intermediate is then reduced by one electron to form the active Mn(IV)/Fe(III) state. The Mn(IV)/Fe(III) cluster can initiate the proton coupled electron transfer to form the cysteine radical in R1 reducing the heterobinuclear center to the Mn(III)/Fe(III) state. The Mn(III)/Fe(III) state can be regenerated to the Mn(IV)/Fe(III) state for further catalysis. In a previous publication we (Younker et al.) utilized the heterobinuclear center of Ct R2 and characterized the structure of the Mn(IV)/Fe(III) cofactor utilizing extended x-ray absorption fine structure (EXAFS) from both the Mn and Fe sides and density functional theory (DFT) calculations. We now wish to utilize these same techniques to structurally characterize the Mn(IV)/Fe(IV) intermediate.